1. Field of the Invention
The present invention relates to axial gap rotating electric machines such as electric generators and electric motors having a permanent magnet.
2. Discussion of the Background
Rotating electric machine apparatus, such as an electric generator for electromagnetically generating electric power by relative rotation of a permanent magnet and an armature, or an electric motor for generating relative rotation between a permanent magnet and an armature by interaction of the permanent magnet and a magnetic field generated by electric current supplied to the armature, are excellent in energy efficiency and have been widely used routinely with development of permanent magnets. However, in both electric motors and electric generators, optimum power is not always obtained in a wide rotational speed range because the amount of magnetic flux supplied from field magnet is constant.
In the case of the electric motor, it will become difficult to control the electric motor in a high-speed rotational region because the back electromotive force (power generation voltage) becomes too high, and therefore, various methods for weakening the field strength as field-weakening control have been proposed. On the other hand, in the case of the electric generator, a constant-voltage electric generator by means of field current control only or a constant-voltage circuit in which the power generation voltage is made to be constant by a semiconductor has been exclusively used so that the power generation voltage is made to be at a predetermined level in a wide rotational-speed range.
In the case of electric motor, field-weakening control by means of phase leading electric current control has been widely adopted, but the energy efficiency and the control range are limited. Attempts have been made to perform the field control in the rotating electric machines by means of a mechanical deviation (For example, U.S. Pat. No. 7,567,006) without sacrificing the high energy efficiency upon the rotating electric machines with a permanent magnet. As the field magnet condition can be maintained in the form of mechanical deviation, a rotating electrical machine can be actualized with high energy efficiency with the energy loss due to field excitation being kept minimum.
The other method for controlling strength of magnetic field to keep the energy loss at minimum is to change a magnetization state of a field magnet irreversibly, and a technical proposition is made in Japanese Patent Laid-Open No. 2008-289300. This proposed technology has a configuration in which a configuration for a current excitation as shown in U.S. Pat. No. 5,682,073 is substituted by a magnet excitation configuration in which magnetization can be changed. However, this has drawbacks such as that the field magnet is susceptible to the magnetic field of an armature coil, configuration thereof is complicated because an excitation coil and the armature coil interfere with each other, and application is limited to short shaft electric rotational machines because magnetic flux from the field magnet concentrate in the central area.
U.S. Pat. No. 5,767,601, U.S. Pat. No. 6,373,162, and Japanese Patent Laid-Open No. Hei 10-271784 have proposed magnetic field controls for electromagnetic rotational devices of the axial gap structure but those show examples of electric current excitation type. In those proposals, the amount of fluxes from adjacent salient poles of a rotor are imbalance, and induced voltage wave form to the armature coils are distorted and the fluctuation of torque are large, and controllable range of the amount of flux is limited. Moreover, concerns remain that the magnetic flux from a permanent magnet in the rotor may be short-circuited by a excitation flux path. In addition, in any proposal mentioned above, half of the rotor surface is a permanent magnet, so it is difficult to make use of a reluctance torque.